According to a conventional optical device including an optical element provided in a housing, a window that is an opening is provided in the housing, so that external light (optical signal) can enter, via the window, the optical element which is provided in the housing. This causes the window to be blocked by a glass member that allows light to pass therethrough.
An optical device disclosed in Patent Literature 1 includes, as an optical element, a wavelength variable interference filter provided in a housing. According to the optical device, (i) a lid is provided at an upper part of the housing and (ii) a glass member is combined with an upper part of the lid by a fixing member. In a center part of the lid, a window that is a circular opening is provided, and a glass member is provided so as to block the window. The fixing member between the lid and the glass member is made of, for example, a low-melting glass, and has a width equal to an overlapping width by which the lid and the glass member overlap each other. That is, the fixing member is configured so that (i) an inner peripheral-side edge part along the window matches an inner peripheral-side edge part of the lid and (ii) an outer peripheral-side edge part matches an outer peripheral-side edge part of the glass member.
An optical device (optical switch module) disclosed in Patent Literature 2 includes, as an optical element, a MEMS (Micro Electro Mechanical System) mirror array chip provided in a housing. According to the optical device, (i) a lid frame is provided at an upper part of a housing and (ii) a glass member (plate-like sapphire) is combined with an upper part of the lid frame by a fixing member. In a center part of the lid frame, a window that is a rectangular opening is provided, and a glass member is provided so as to block the rectangular opening. The fixing member between the lid frame and the glass member has a width narrower than an overlapping width by which the lid frame and the glass member overlap each other. The fixing member is configured so that an inner peripheral-side edge part along the opening matches an inner peripheral-side edge part of the lid frame.
[Patent Literature 1]
Japanese Patent Application Publication Tokukai No. 2015-31903 (Publication date: Feb. 16, 2015)
[Patent Literature 2]
Japanese Patent Application Publication Tokukai No. 2011-8105 (Publication date: Jan. 13, 2011)
FIG. 10 is a perspective view illustrating main components of a conventional optical device 101 including an optical element in a housing and being configured so that (i) a window 111a that is an opening is provided in a frame 111 constituting the housing and (ii) the window 111a is blocked by a window glass plate 112. (a) of FIG. 11 is a cross-sectional view taken along the line D-D in FIG. 10. (b) of FIG. 11 is a cross-sectional view illustrating cracking of a glass member illustrated in (a) of FIG. 11. (c) of FIG. 11 is a plan view illustrating the parts illustrated in (b) of FIG. 11. Note that the window glass plate 112 is provided in the frame 111 so as to block the window 111a from below.
According to the configuration in which the window 111a of the frame 111 is blocked by the window glass plate 112, a solder (Au—Sn) is widely used as a fixing member for fixing the window glass plate 112 to the window 111a of the frame 111 which is made of metal. Specifically, as illustrated in (a) of FIG. 11, (i) a metal coating film 114 is annularly formed in a region which is located in a periphery of the window glass plate 112 and in which a solder layer (fixing member) 113 is provided and (ii) a solder layer 113 is provided on the metal coating film 114. Therefore, the window glass plate 112 is fixed to the frame 111 by the solder layer 113 between the metal coating film 114 and the frame 111. That is, the window glass plate 112 is fixed to the frame 111 by the solder layer 113 being bonded to the metal coating film 114 and to the frame 111 by metallic bond.
In this case, in a case where the solder layer 113 is melted, the solder layer 113 is wetly spread all over a top surface of the metal coating film 114. Therefore, in a case where an inner peripheral-side edge part 114a of the metal coating film 114 protrudes toward a center part of the window 111a further than a window-side edge part 111b of the frame 111, cracking 112a of the window glass plate 112 occurs (see (b) of FIG. 11) when a solder of the solder layer 113 having been melted is cooled and therefore hardened. This is because of a difference in thermal expansion coefficient between the solder of the solder layer 113 and the window glass plate 112. In other cases, after the melted solder of the solder layer 113 is cooled and therefore hardened, the difference in thermal expansion coefficient between the solder of the solder layer 113 and the window glass plate 112 results in strain of the window glass plate 112. This causes the occurrence of the cracking 112a of the window glass plate 112. In this case, the optical device 101 becomes less hermetic and durable.
Specifically, in a case where the solder layer 113 is cooled and therefore hardened, the solder layer 113 shrinks. The shrinkage of the solder layer 113 becomes notable in a region where no part of the frame 111 is located on the solder layer 113. This is because the shrinkage of the solder is not reduced by the frame 111 in the region. Therefore, in the region where no part of the frame 111 is located on the solder layer 113, the shrinkage of the solder layer 113 causes tensile stress, which is generated in a direction toward an outer peripheral-side end part of the metal coating film 114, to be applied to the metal coating film 114, that is, to the window glass plate 112 on which the metal coating film 114 is provided. This causes the occurrence of the cracking 112a of the window glass plate 112.
In addition, an inner peripheral-side edge part 113a of the solder layer 113 tends to protrude, toward the center part of the window 111a, further than the inner peripheral-side edge part 114a of the metal coating film 114. This causes an opening area of the window 111a to be small.
Note that according to the configurations of Patent Literatures 1 and 2, the glass member is fixed to the frame by the fixing member provided between (i) the frame (lid or lid frame) constituting the housing and (ii) the glass member. Furthermore, the inner peripheral-side edge part (corresponding to the inner peripheral-side edge part 114a) of the fixing member matches the window-side edge part (corresponding to the window-side edge part 111b) of the frame. According to this configuration, a tolerance during production of the optical device causes the inner peripheral-side edge part of the fixing member to easily protrude, in a direction toward the center part of the window, further than the inner peripheral-side edge part of the frame, which inner peripheral-side edge part is located along the window. Specifically, according to the configurations of Patent Literatures 1 and 2, in a case where a solder and a metal coating film are used as a fixing member, the inner peripheral-side edge part of the metal coating film protrudes, to the center part of the window, further than the window-side edge part of the frame as with the configuration illustrated in (a) and (b) of FIG. 11. This likewise causes the occurrence of cracking of the window glass plate.